The present invention relates to acquisition of electrical signals, and more particularly to zero dead time acquisition for a digital storage oscilloscope (DSO) that allows a conventional flash conversion DSO to perform tasks normally only available in analog oscilloscopes.
An analog oscilloscope displays an analog signal in real time as a function of amplitude versus time. The display of the analog signal starts when a trigger signal is generated in response to a trigger event, such as the zero crossing of the analog signal. The trigger signal is used to start the trace of an electron beam from one edge of a display area across the faceplate of a cathode ray tube (CRT). The analog signal is used to modulate the trace in an orthogonal direction to produce the familiar amplitude versus time display. If a portion of the analog signal before the trigger signal is desired to be displayed, the analog signal may be delayed. When the trace reaches the opposite edge of the display area, a hold-off time is used to shut off the beam during the electron beam retrace time. The time that the electron beam takes to sweep across the display area is the display time. The next sweep of the oscilloscope occurs on the next trigger signal after the hold-off time to start the next display time.
A digital storage oscilloscope (DSO) does not display the analog signal in real time. Rather the DSO samples and digitizes the analog signal at a predetermined sample rate to produce a stream of digital data samples that represent the analog signal. The digital data samples are processed and eventually stored in a main memory, from which they may be read out for display to reproduce the analog signal. There are two modes of acquiring a digital representation of the analog signal--a post-trigger mode and a pre-trigger mode. The post-trigger mode is equivalent to the analog counterpart in that no data samples are acquired until the trigger event generates the trigger signal. After the trigger event occurs the analog signal is sampled and digitized and the resulting digital data samples are stored in sequential locations of an acquisition memory, ignoring for the purpose of this discussion equivalent and random time sampling techniques used for very high frequency repetitive signals, until the acquisition memory is "full". One way to provide greater accuracy for the data that is displayed in the DSO is to oversample the analog signal so that, if the acquisition memory has 500 locations, 4000 samples of the analog signal may be obtained and decimated into 500 samples for storage, as an example. Each location in the acquisition memory represents a sample interval, with eight subsamples for the present example. The subsamples are processed in real time to obtain a single value for the sample interval or, in the case of an envelope mode, a minimum and a maximum value for the sample interval. Due to the length of time that it requires to digitally process the data in the acquisition memory before it is transferred into the main memory, such as determining minimum and maximum values between a prior acquisition cycle stored in the main memory and the just acquired data samples from the acquisition memory, the digital processing time may be considerably longer than the analog hold-off time.
If a spurious impulse occurs sporadically in the analog signal, conventional wisdom is that it is more likely to be observed by the analog oscilloscope, especially if it is of the micro-channel plate (MCP) type, than by a DSO. Two factors enter into this--the shorter hold-off time for analog oscilloscopes and the time between data samples of the digital oscilloscope. The period of time during which the analog signal is not displayed (analog oscilloscope) or acquired (DSO) is termed "dead time." In a digital storage oscilloscope (DSO), for example, normal MIN/MAX decimation systems start with a MIN of "ff" and a MAX of "00" (8-bit system) for a sample interval of interest. All subsamples within the sample interval are compared with the stored MIN/MAX values, and replace the stored values if necessary as indicated by the comparison. The final MIN/MAX pair for the interval is then stored in an acquisition memory, the location corresponding to the sample interval. When an acquisition cycle is complete, the contents of the acquisition memory and a previous copy in a main memory are MIN/MAX'd together by a processor, with the results being replaced in the main memory. The time it takes for the processor to perform this operation between the main and acquisition memories is considerable when compared to the hold-off time of an analog oscilloscope, which contributes to dead time. Even if a redundant acquisition memory is provided, the processing time still takes too long so that the redundant acquisition memory is filled before the processing is completed. The dead time is merely reduced, but not eliminated.
DSOs also have the ability to display data prior to the trigger signal in a pre-trigger mode, which has the capability of providing much more of the analog signal for display that occurs before the trigger signal than does the analog signal delay of the analog oscilloscope. In fact in the extreme the only data displayed may be all pre-trigger data. The pre-trigger time needs to expire before the trigger signal causes the DSO to start the next acquisition cycle.
What is desired is a means for implementing a "zero dead time" acquisition mode that performs decimation and accumulation at acquisition time while eliminating the necessity of having a hold-off time due to digital signal processing.